The DC output from a sound level meter or noise processor provides a voltage level that is proportional to the decibel level measured by the meter. The audio signal is generally processed (time and frequency weighted) before being output as a DC level.
The AC output is used for applications that need access to the unprocessed, but amplified, original waveform. This is useful for recording devices and for signal sampling and analysis. The AC signal will be unfiltered and unweighted.